Lithium ion secondary batteries and other nonaqueous electrolyte secondary batteries are of increasing importance as on-board power sources for vehicles and power sources for personal computers and handheld devices. In particular, lithium ion secondary batteries, which are lightweight and can achieve a high-energy density, are desirable as high-output on-board power sources for vehicles.
However, in nonaqueous electrolyte secondary batteries such as lithium ion secondary batteries, some of the nonaqueous electrolyte solution decomposes during charging, and a film made up of the products of such decomposition, known as a solid electrolyte interface (SEI) film, may form on surfaces of the negative electrode active material (e.g., natural graphite particles). Although a SEI film does serve to protect the negative electrode active material, formation of the film involves the consumption of charge carriers (e.g., lithium ion) within the nonaqueous electrolyte solution. That is, because the charge carriers are immobilized within the SEI film, they can no longer contribute to the battery capacity. Therefore, the formation of a large amount of SEI film is a major cause of declines in the capacity ratio (declines in the cycle characteristics).
To address this problem, various types of additives have been included in the nonaqueous electrolyte solution so as to form beforehand a stable film on the surface of the negative electrode active material instead of a SEI film. For example, Patent Literature 1 discloses a nonaqueous electrolyte solution for secondary batteries which contains lithium bis(oxalato)borate (Li[B(C2O4)2]) as an additive.